This invention relates generally to transfer cases and more particularly to two speed transfer cases that include a speed change gear set to provide low speed range and high speed range operation.
In general, there are two basic types of power transfer mechanisms, such as transfer cases, that are operatively associated with both manual and automatic transmissions for directing power to the four wheels of a motor vehicle. These are the full time or all wheel drive type in which all four wheels of the vehicle are driven and the part time type in which two wheels are normally driven and the other two wheels are selectively driven.
Many automotive transfer cases, particularly those of the part time type, are equipped with a speed change gear set or the like for operating in low and high speed ranges in conjunction with two wheel drive and four wheel drive operating modes. These transfer cases include a shift mechanism for moving a shift sleeve or the like back and forth between a high speed range position that usually provides direct drive and a low range speed position that usually provides reduced ratio drive. See for instance U.S. Pat. No. 5,159,847 granted to Randolph C. Williams Nov. 3, 1993 for a sector plate for a transfer case. The shift mechanism of this particular transfer case cannot be operated unless the vehicle is stopped. However, it is known that mechanical synchronizers can be incorporated into two speed transfer cases to allow dynamic shifting while the vehicle is in motion.
However transfer cases are rotationally secured to the output shaft of the vehicle transmission and therefore have a relatively high rotational inertia reflected to the input shaft of the transfer case. This high reflected moment of inertia combined with the limited torque capacity of a mechanical synchronizer limits dynamic shifting to very low vehicle speeds.
On the other hand, many transfer cases, particularly those of the full time type, are equipped with a differential that allows the outputs to turn at different speeds. These differentiated transfer cases split the input torque between the respective outputs for the front and rear wheels of the automobile according to the physical characteristics of the differential. However, it is also known that the torque split between the two outputs can be modulated to meet certain operating conditions. U.S. Pat. No. 4,718,303 granted to Mark J. Fogelberg Jan. 12, 1988 discloses such a differentiated transfer case which includes an electromagnetic friction clutch which operates on the two outputs to modulate the torque split between the front and rear drive axles of the vehicle. This modulatable friction clutch allows the transfer case to easily and readily adapt to changing vehicle conditions without detracting from its four wheel drive capabilities. Also see U.S. Pat. No. 4,989,686 granted to Alan L. Miller et al Feb. 5, 1991 for a system for controlling torque transmission in a four wheel drive vehicle.
Another type of transfer case is the on demand type which can be considered a species of the part time type. In this type two wheels are normally driven and the other two wheels are automatically driven in response to an external stimulus such as slippage of the normally driven wheels. These on demand type transfer cases customarily include a friction type clutch, such as a viscous coupling connecting two elements of the differential that allows the two outputs to turn at different speeds up to a predetermined speed differential according to the physical characteristics of the friction clutch.
The torque split between the two outputs of these on demand type transfer cases can be modulated by using an electromagnetic friction clutch as disclosed in U.S. patent application Ser. No. 07/903,696 filed Jun. 24, 1992, now U.S. Pat. No. 5,407,024, and U.S. patent application Ser. No. 08/100,560 filed Jul. 30, 1993, now U.S. Pat. No. 5,409,429, both of which are hereby incorporated in this patent specification by reference.
It should also be noted that the electromagnetic friction clutch is incorporated in the transfer case of U.S. patent application Ser. No. 08/100,560 so as to permit dynamic upshifts to an on demand high speed range operating system.